Inverted-F antennas are used in wireless communications systems including mobile telephones, pagers, Global Positioning System (GPS), wireless LAN, WiFi, aircraft, locomotives, vehicles, radiolocation devices etc. Inverted-F antennas typically include a linear (i.e., straight) conductive element, e.g. a wire, that is maintained in spaced apart relationship with respect to a ground plane. They are especially useful where a low profile antenna is needed, one that does not stand tall above communications device, or mobile platform.
The inverted-F antenna is essentially a shunt fed inverted-L antenna, excited by a feed tap. That is, the antenna is tapped at a distance from the base to provide a desired driving point resistance. Thus, any resistance level may be obtained by adjusting the tap position. The inverted-F antenna may be preferred to the inverted-L antenna because it may not need an external matching network and may allow independent adjustment of the resonant frequency of the antenna and resistance level. In general, the overall length of the inverted-F antenna (height plus length) is approximately one-quarter wavelength at the resonant frequency.
The planar inverted-F antenna (PIFA) is a derivative of the inverted-F antenna in which the top wire is coplanar with a plate. This has been shown to lower the radiation Q and thus broaden the frequency response of the antenna while still retaining the desirable characteristics of the wire inverted-F antenna. PIFAs are typically used within wireless communication devices where externally mounted antennas are less desirable.
Conventional inverted-F antennas, by design, resonate within a narrow frequency band, as compared with other types of antennas, such as helices, monopoles and dipoles. Examples of inverted-F antennas are described in U.S. Pat. Nos. 5,684,492 and 5,434,579, for example.
The feed structure is typically a coaxial cable, and for inverted-F antennas that are mountable on a surface, such as on a vehicle, the feed must penetrate the surface to be connected to the antenna. This may make mounting difficult on some surfaces.